Led thermal characterization and calibration for an optical display

ABSTRACT

Techniques are disclosed for calibrating display devices, such as head-mounted augmented reality (AR) devices. For each of a set of primary color light sources disposed in a unit, measurement points are obtained including chromaticity and luminance values as a function of temperature of the unit and drive currents of the primary color light sources. The measurement points are converted to an XYZ color space to produce XYZ values. For each of a set of target luminances and a set of discrete temperatures, primary luminances are calculated for each of the set of primary color light sources, and the drive currents and the XYZ values are interpolated or extrapolated for each of the primary luminances to obtain calculated currents.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/530,599, filed on Aug. 2, 2019, entitled “LED THERMALCHARACTERIZATION AND CALIBRATION FOR AN OPTICAL DISPLAY,” which is anon-provisional of and claims the benefit of and priority to U.S.Provisional Patent Application No. 62/714,503, filed Aug. 3, 2018,entitled “LED THERMAL CHARACTERIZATION AND CALIBRATION FOR AN OPTICALDISPLAY,” which are hereby incorporated by reference in their entiretyfor all purposes.

BACKGROUND OF THE INVENTION

Modern computing and display technologies have facilitated thedevelopment of systems for so called “virtual reality” or “augmentedreality” experiences, wherein digitally reproduced images or portionsthereof are presented to a user in a manner wherein they seem to be, ormay be perceived as, real. A virtual reality, or “VR,” scenariotypically involves presentation of digital or virtual image informationwithout transparency to other actual real-world visual input; anaugmented reality, or “AR,” scenario typically involves presentation ofdigital or virtual image information as an augmentation to visualizationof the actual world around the user.

Despite the progress made in these display technologies, there is a needin the art for improved methods, systems, and devices related toaugmented reality systems, particularly, display systems.

SUMMARY OF THE INVENTION

The present disclosure relates generally to techniques for calibratingdisplay devices. More particularly, embodiments of the presentdisclosure provide systems and methods for performing a thermalcharacterization and calibration of one or more light-emitting diodes(LEDs) in an augmented reality (AR) device. Although portions of thepresent disclosure are described in reference to an AR device, thedisclosure is applicable to a variety of applications in image displaysystems.

In accordance with a first aspect of the present invention, a method ofcalibrating a display device including a set of primary color lightsources disposed in a unit is provided. The method may includeobtaining, for each of the primary color light sources, measurementpoints including chromaticity and luminance values as a function oftemperature of the unit and drive currents of the primary color lightsources. The method may also include converting the measurement pointsto an XYZ color space to produce XYZ values. The method may furtherinclude, for each of a set of target luminances and a set of discretetemperatures, calculating primary luminances for each of the primarycolor light sources and interpolating or extrapolating the drivecurrents and the XYZ values for each of the primary luminances to obtaincalculated currents. The method may further include storing thecalculated currents indexed by the target luminances and the discretetemperatures.

In some embodiments, the method further includes interpolating orextrapolating the measurement points to provide modified measurementpoints as a function of the discrete temperatures and the drivecurrents. In some embodiments, the method further includes, for each ofthe discrete temperatures, sorting the modified measurement points bythe luminance values. In some embodiments, the primary color lightsources are LEDs. In some embodiments, the unit comprises a compoundparabolic concentrator (CPC). In some embodiments, measuring thechromaticity and luminance values comprises receiving light transmittedthrough an eyepiece of the display device. In some embodiments,calculating the primary luminances and interpolating or extrapolatingthe drive currents and the XYZ values are performed over multipleiterations. In some embodiments, the calculated currents are stored in alookup table.

In accordance with a second aspect of the present invention, a systemfor calibrating a display device including a set of primary color lightsources disposed in a unit is provided. The system may include one ormore processors. The system may also include a computer-readable mediumcomprising instructions that, when executed by the one or moreprocessors, cause the one or more processors to perform operations. Theoperations may include obtaining, for each of the primary color lightsources, measurement points including chromaticity and luminance valuesas a function of temperature of the unit and drive currents of theprimary color light sources. The operations may also include convertingthe measurement points to an XYZ color space to produce XYZ values. Theoperations may further include, for each of a set of target luminancesand a set of discrete temperatures, calculating primary luminances foreach of the primary color light sources and interpolating orextrapolating the drive currents and the XYZ values for each of theprimary luminances to obtain calculated currents. The operations mayfurther include storing the calculated currents indexed by the targetluminances and the discrete temperatures.

In some embodiments, the operations further include interpolating orextrapolating the measurement points to provide modified measurementpoints as a function of the discrete temperatures and the drivecurrents. In some embodiments, the operations further include, for eachof the discrete temperatures, sorting the modified measurement points bythe luminance values. In some embodiments, the primary color lightsources are LEDs. In some embodiments, the unit comprises a CPC. In someembodiments, measuring the chromaticity and luminance values comprisesreceiving light transmitted through an eyepiece of the display device.In some embodiments, calculating the primary luminances andinterpolating or extrapolating the drive currents and the XYZ values areperformed over multiple iterations. In some embodiments, the calculatedcurrents are stored in a lookup table.

In accordance with a third aspect of the present invention, anon-transitory computer-readable medium is provided. The non-transitorycomputer-readable medium may include instructions that, when executed byone or more processors, cause the one or more processors to performoperations including: obtaining, for each of a set of primary colorlight sources disposed in a unit of a display device, measurement pointsincluding chromaticity and luminance values as a function of temperatureof the unit and drive currents of the primary color light sources. Theoperations may also include converting the measurement points to an XYZcolor space to produce XYZ values. The operations may further include,for each of a set of target luminances and a set of discretetemperatures, calculating primary luminances for each of the primarycolor light sources and interpolating or extrapolating the drivecurrents and the XYZ values for each of the primary luminances to obtaincalculated currents. The operations may further include storing thecalculated currents indexed by the target luminances and the discretetemperatures.

In some embodiments, the operations further include interpolating orextrapolating the measurement points to provide modified measurementpoints as a function of the discrete temperatures and the drivecurrents. In some embodiments, the operations further include, for eachof the discrete temperatures, sorting the modified measurement points bythe luminance values. In some embodiments, the primary color lightsources are LEDs.

Numerous benefits are achieved by way of the present disclosure overconventional techniques. For example, embodiments described hereinenable brightness and color matching of displays between depth planes,between eyes, and between units. Embodiments described herein allowcolor calibration while eliminating the need to improve the accuracy ofthe color sources themselves by instead characterizing the actualspectral characteristics of the color sources and adjusting thecombination amounts of each color based on the characterization. Otherbenefits of the present disclosure will be readily apparent to thoseskilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic view of a display device according to anembodiment of the present invention.

FIG. 2 illustrates a schematic diagram of a spectrometer calibrationsystem for a display device according to an embodiment of the presentinvention.

FIG. 3 illustrates an example characterization of a red LED.

FIG. 4 is a simplified flowchart illustrating a method of setting LEDcurrents using a PID controller according to an embodiment of thepresent invention.

FIG. 5 illustrates an example characterization of a green LED.

FIG. 6 illustrates an example characterization of a blue LED.

FIG. 7 illustrates a method for characterizing primary LEDs of anoptical system such as a display device according to an embodiment ofthe present invention.

FIGS. 8A-8C illustrate a method of characterizing primary color LEDs ofan optical system according to an embodiment of the present invention.

FIG. 9 illustrates a operation of a first set of primary LEDs of adisplay device that have been previously characterized to generate a 2DLUT.

FIG. 10 illustrates a operation of a second set of primary LEDs of adisplay device that have been previously characterized to generate a 2DLUT.

FIG. 11 illustrates a method for calibrating a display device.

FIG. 12 illustrates a block diagram of a feed forward controlleraccording to an embodiment of the present invention.

FIG. 13 illustrates a simplified computer system according to anembodiment described herein.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The brightness and color of an optical display being powered bylight-emitting diodes (LEDs) are highly sensitive to the spectralcharacteristics of the LEDs themselves. For example, due to bothfabrication variability and the inherent limitations of narrow-bandLEDs, the spectral characteristics of an LED can deviate significantlyfrom an ideal monochromatic light source. The problem is exacerbated foroptical systems, such as an optical see-through head-mounted display(OST-HMD), which include various optical components through which thelight is channeled before arriving at the eye of the user. Eachcomponent may apply a unique distortion to the light that is difficultto model and/or account for. For displays that generate a particularcolor point by combining varying amounts of primary colors (e.g., red,green, and blue), the accuracy of the color point can be improved by (1)improving the accuracy of the primary colors themselves or by (2)characterizing the actual spectral characteristics of the color sourcesand adjusting the combination amounts of each color based on thecharacterization. Embodiments of the present invention are directed tothe latter approach.

Embodiments of the present invention improve the brightness and color ofoptical displays by first characterizing the temperature and currentdependence of each source LED and second adjusting the way the displayreceives and handles the emitted light from the source LEDs based on thecharacterization. These two steps may collectively be referred to as a“thermal calibration” or a “color calibration” of the optical display.The characterization step may occur during or after fabrication, e.g.,while the device is still in the factory, by positioning a lightdetection device (e.g., a spectrometer) at a distance from the eyepiecewhere the user's eye would receive the light when the device is in use.The light detection device sequentially detects each source LED's outputthrough the optical device as a controller (e.g., feed forwardcontroller) iterates through a plurality of temperatures and currents.The outputs are analyzed to extract chromaticity and luminance datawhich are used to characterize each source LED. When the device is inactual use by the user, the calibration step is performed by measuring atemperature and adjusting the LED current and the color computationalgorithm based on the characterization data as informed by the measuredtemperature.

FIG. 1 illustrates a schematic view of a display device 100 according toan embodiment of the present invention. Display device 100 may be anynumber of optical systems or display devices, such as head-mounteddisplays including augmented reality (AR) devices, virtual reality (VR)devices, and the like. Display device 100 may include a left eyepiece102A and a right eyepiece 102B each containing multiple waveguides 103arranged in a side-by-side configuration forming an optical stack. Eachof waveguides 103 receives light from a particular source LED that isspatially modulated by a liquid crystal on silicon (LCoS) projector 104.For example, referring to the right eyepiece 102B, the waveguidecorresponding to red and depth plane 1 (i.e., waveguide R1) may receivelight from the LED within a LED unit 106B corresponding to red and depthplane 1 (i.e., LED-R1) after the light is spatially modulated by LCoSprojector 104B. Similarly, as another example, the waveguidecorresponding to blue and depth plane 2 (i.e., waveguide B2) may receivelight from the LED within LED unit 106B corresponding to blue and depthplane 2 (i.e., LED-B2) after the light is spatially modulated by LCoSprojector 104B. The disclosure is applicable to any number of opticalsystems or displays. Accordingly, LCoS projectors 104 may be configuredso as to channel the separate light inputs to the separate waveguides ofeyepieces 102 without combining or mixing the light signals. AlthoughFIG. 1 is described in reference to LCoS projectors 104, other types ofspatial light modulators may be used.

In some embodiments, display device 100 includes a controller 108 forsending display instructions to LCoS projectors 104 and supplying drivecurrent(s) 110 to the source LEDs. The display instructions may indicatewhich source LEDs are to be modulated and the modulation settings. Insome embodiments, LCoS projectors 104 may sequentially modulate thelight from different source LEDs (i.e., one at a time in a mannerconsistent with a field sequential display) or in some embodiments thelight from multiple source LEDs may be modulated simultaneously.Controller 108 may also supply drive current(s) 110 to one or more ofthe source LEDs in coordination with the display instructions. In someembodiments, the display instructions and drive current(s) 110 may beinfluenced by temperatures 112 measured using thermistors coupled to LEDunits 106. Temperatures 112 may correspond to a temperature of LED units106 which may be used as an approximation of the temperatures of thesource LEDs. Temperatures 112B may be measured at predeterminedintervals or in response to satisfaction of one or more criteria. Forexample, controller 108 may retrieve temperatures 112 each time a newdisplay instruction is sent to LCoS projectors 104 and/or each time auser requests a recalibration of display device 100. Although FIG. 1A isdescribed in reference to thermistors, other types of temperaturesensors may be used.

In some embodiments, each of LED units 106 may comprise a thermalconductor coupled to each of the LEDs contained in the particular unitsuch that heating occurring at one LED may influence the temperature ofother LEDs that are part of the same LED unit. Each of LED units 106 mayinclude six LEDs (as illustrated in FIG. 1A), or some embodiments, eachof LED units 106 may include three LEDs corresponding to primary colorsof a particular depth plane. In some embodiments, multiple temperaturesensors may be coupled to LED units 106 so as to better approximate theactual temperature of a particular LED. In some embodiments, each of LEDunits 106 may comprise one or more optical concentrators to collimateand/or modify the light emitted by the LEDs, for example, a CompoundParabolic Concentrator (CPC).

FIG. 2 illustrates a schematic diagram of a spectrometer calibrationsystem 200 for display device 100 according to an embodiment of thepresent invention. Referring to FIG. 2, display device 100 is positionedsuch that light emitted from waveguides 103 of eyepieces 102 arecaptured by collimating lenses 216 or other suitable optical couplingdevices. As illustrated in FIG. 2, at least a portion of the light fromeach waveguide is captured and can be collimated for delivery to acharacterization fiber 218. Characterization fibers 218 are opticallycoupled to spectrometers 214 that are utilized to characterize thespectral properties of the light emitted from the waveguides asdescribed more fully herein. A calibration computer 202 may be utilizedto control spectrometer calibration system 200.

During calibration, calibration computer 202 may instruct display device100 to generate a virtual image that is displayed by display device 100.The term virtual image includes the display of a uniform screen of lightassociated with one of the primary colors, that is, a red screen, agreen screen, or a blue screen. As an example, calibration computer 202can control display device 100 to display light from a red LED. In thisexample, controller 108 may communicate with the red LED to generate anoutput. In some instances, the SLM drive electronics can be used to setthe SLM in the projector to provide a constant reflection (e.g., maximumreflection) across the SLM in the projector. As a result, the lightgenerated by the red LED will couple into and be emitted by waveguide103, be collected by collimating lens 216, and be coupled intospectrometer 214 through fiber 218. Both the right and left red LEDs canbe characterized either concurrently (e.g., simultaneously) orsequentially. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives.

In addition to using the LEDs in display device 100 to generate lightthat will enable spectral characterization of the LEDs, the LEDs can beutilized to generate heat in LED units 106, enabling characterization ofthe LEDS as a function of both drive current and CPC temperature.Additional description related to temperature control of LED units 106is provided herein.

As described herein, embodiments of the present invention enable thespectral characterization of the LEDS in display device 100 in anassembled state of the wearable. Although the spectral characteristicsof the LEDs in the LED package can provide insight into the LEDcharacteristics in a discrete package, this characterization does nottake into account optical impacts of incorporating the LEDs into theassembled wearable. For example, optical filters disposed along theoptical path between the LEDs and the waveguide can absorb predeterminedportions of the light emitted by the LEDS. As a result, the portions ofthe spectrum absorbed by the spectral filter will result in the spectralcharacteristics of the light emitted by the waveguides differing fromthe spectral characteristics of the light as emitted from the LEDpackage. Moreover, the spectral throughput properties of the elements ofthe eyepiece, including the waveguide, can modify the spectralcharacteristics of the light after emission from the LED package.Accordingly, because the spectrometers receive light emitted by theassembled wearable, the spectral characterization of the LEDs includesthe optical throughput properties of the various optical elements in theassembled wearable, not the spectral characteristics of the LED packagebefore integration into the assembled wearable.

FIG. 3 illustrates an example characterization of LED-R1. In someembodiments, a light detection device (e.g., spectrometer 214) ispositioned at a particular distance from eyepiece 102B at a positionthat is approximately where the user's eye (also referred to as aviewer's eye) would receive the light when the device is in use.Spectrometer 214 may receive light through a collimating lens 216 thatreceives the light emitted by Display device 100 and a fiber 218 thatdirects the received light to spectrometer 214. In some embodiments,spectrometer 214 may be configured to measure the chromaticity and theluminance of the received light (e.g., in the xyY color space). This maybe accomplished either by measuring the xyY values directly or by firstmeasuring the light output as a function of wavelength and secondanalyzing the spectral characteristics of the light output to determinethe xyY values.

Embodiments of the present invention characterize the spectral contentof the light sources, e.g., LEDS, after propagation through the fulloptical stack of the optic device, for example, a wearable AR device.Accordingly, all of the optical characteristics, spectral attenuations,efficiencies, and the like are considered in calibrating the lightsources, e.g., LEDs.

During the characterization of LED-R1, proportional-integral-derivative(PID) controller 132 causes the current powering LED-R1, i_(R) of drivecurrent(s) 110B, to iterate through a plurality of discrete currents.The discrete current values may be sent one at a time from the PIDcontroller 132 to an LED current controller 122. LED current controller122 may gradually ramp up or down current i_(R) toward the particulartarget current or may, in some embodiments, immediately set currenti_(R) to the target current. In addition to current control of currenti_(R), LED current controller 122 may also provide one or moreadditional currents to the other illustrated LEDs that, as describedmore fully below, can cause temperature 112 of LED unit 106 to varyacross a range of temperatures. Temperature control of the LED Unit canthus be achieved by controlling the currents of the non-characterizedLEDs. For example, LED current controller 122, working in conjunctionwith PID controller 132, may increase or decrease the current values forcurrents i_(G) and i_(B) so as to increase or decrease temperature 112of LED unit 106. As an example, as temperature 112 is fed back into PIDcontroller 132, a comparison can be made between the previoustemperature and the current temperature to determine how much each ofthe currents i_(G) and i_(B) need to be increased or decreased.

In order to measure the xyY values of the light output of LED-R1 acrossa representative range of both current i_(R) and temperature 112, PIDcontroller 132 may cause temperature 112 to vary while maintainingcurrent i_(R) constant. Alternatively or additionally, PID controller132 may cause current i_(R) to iterate through a plurality of discretecurrents while maintaining temperature 112 relatively constant. In someembodiments, PID controller 132 may cause current i_(G) and temperature112 to vary concurrently or simultaneously. For example, PID controller132 may drive current i_(G) and temperature 112 toward a desiredcurrent-temperature pair of a list of current-temperature pairs. PIDcontroller 132 may determine a fastest or most power efficienttrajectory to navigate through the list of current-temperature pairs.Other possibilities are contemplated.

During the characterization of LED-R1, synchronization betweenspectrometer 214 and processing unit 130 (which is thereby synchronizedwith PID controller 132 and LED current controller 122) is utilized suchthat the LCoS projector 104 reflects only light output from LED-R1 foreventual delivery through eyepiece 102. As described below, one or moreof the LEDs (e.g., all the LEDs) can be driven at specific currentlevels to generate the desired LED Unit temperature 112. Once thedesired temperature is achieved, the LED being characterized can beoperated to illuminate LCoS projector 104 for a predetermined timeperiod.

Referring to FIG. 3, the target current for the red LED (i_(R)) and thetarget temperature of LED unit 106 are defined. The drive currentsapplied to each of the LEDs in LED unit 106 can be predetermined drivecurrents (i.e., set drive currents). The target temperatures of LED unit106 can also be predetermined. Accordingly, as described more fullybelow, the chromaticity and luminance associated with each of the LEDscan be measured at a range of drive currents and LED unit temperatures.

For a field sequential display, the light sources (e.g., three of theRGB LEDs illustrated in FIG. 3) are driven sequentially insynchronization with the LCoS projector 104. For example, at 120 Hz, theframe time is ˜8.3 ms. Each LED associated with a depth plane of theeyepiece (e.g., LED-R1, LED-G1, and LED-B1) may be illuminatedsequentially for ˜2.7 ms.

FIG. 4 is a simplified flowchart illustrating a method 400 of settingLED currents using a HD controller according to an embodiment of thepresent invention. As illustrated in FIG. 4, a target temperature forthe LED Unit is received by the HD controller (step 410). Identifyinginformation related to an LED to be characterized and/or calibrated isalso received by the PID controller (step 412 ). As an example, for anLED unit including two sets of LEDs (a first set of R1, G1, B1, and asecond set of R2, G2, and B2), information can be received that the redLED of the first set, (i.e., LED-R1) is going to be characterized and/orcalibrated. The LED identified for characterization/calibration can bereferred to as the identified LED to differentiate this LED from theother LEDs in the LED unit. In addition to the identifying information,the target drive current for the LED to be characterized/calibrated isreceived (step 414).

In preparation for measurements related to the LEDcharacterization/calibration, in some embodiments, the drive current forall of LEDs in one of the sets, or optionally, all of the LEDs in bothsets, are controlled by the HD controller such that the PID controlleradjusts the drive currents, which can include increasing the drivecurrent(s) provided to one of more of the LEDs to a maximum value orreducing the drive currents(s) to zero, to achieve a LED unittemperature within a predetermined range (e.g., +/−0.5° C.) of thetarget temperature. As described herein, when the LED unit temperatureis within the predetermined range of the target temperature, all LEDsother than the identified LED are turned off, the identified LED isdriven at the target drive current, and the light output of theidentified LED is measured using the spectrometer. During this spectralmeasurement, the LED unit temperature is measured, the averagetemperature during the spectral characterization is determined, and theaverage temperature is recorded, for example, in the same database, forexample, the same entry, as the chromaticity and luminance.

The HD controller communicates with the LED current controller to setthe drive current for the identified LED to the target drive current(step 416). In some embodiments, the temperature of the LED unit can beset at the target temperature independent of thecharacterization/calibration process. Accordingly, steps 412, 414, 416,and 440 are illustrated as optional in FIG. 4. The HD controllerreceives a temperature of the LED unit using the thermistor (step 418),which can be referred to as a measured temperature, and compares thetarget temperature to the measured temperature (step 420). If themeasured temperature is less than the target temperature, then the heatgenerated by one or more of the other LEDs in the LED unit will be usedto increase the temperature of the LED unit. Accordingly, if themeasured temperate is less than the target temperature, then a drivecurrent is determined for an additional LED (step 422). As an example,an initial, low drive current value can be determined for a second LEDin the LED unit, for example, LED-G1. This initial drive current valueis then provided to the LED current controller so that the additionalLED (e.g., LED-G1) is driven at the initial drive current value (step424). The temperature of the LED unit is measured (step 418) and if themeasured temperature is still below the target temperature, then theinitial drive current value for the second LED can be increased to ahigher value (step 422). This process is repeated until the maximumdrive current of the second LED has been reached.

The iterative process of comparing the measured temperature to thetarget temperature is repeated by determining drive currents foradditional LEDs, for example, LED-B1, and the LEDs from the second set,until the target temperature is reached. In this process, each LED caninitially be driven at a low current value that is increased over timeto increase the amount of heat generated by the LED. By increasing thedrive current in each additional LED and then adding each additionalLED, the temperature of the LED unit can be increased in a substantiallylinear manner until the target temperature is reached.

As illustrated in FIG. 4, if the measured temperature is greater thanthe target temperature (step 430), then the drive currents in theadditional LED(s) are decreased by the LED current controller (steps 432and 434) to result in reduced heat generation and decreased LED Unittemperature as a consequence. Thus, if an initial calibration isperformed at a first temperature and a second calibration is performedat a second temperature less than the first temperature, the system candecrease the temperature of the LED unit until the second temperature isreached. Accordingly, embodiments of the present invention enablemeasurement over a range of target temperatures for the LED unit.

Once the LED unit temperature is equal to the target temperature, thespectral characteristics of the identified LED can be measured (step440). In some embodiments, all additional LEDs that were used to achievethe desired target temperature can be turned off. Since the thermal massof the LED unit is non-zero, the LED unit will maintain the targettemperature during the short time interval (several milliseconds)associated with the spectral characterization/calibration.

In some embodiments, the PID controller can include logic that willenable the iterative loop to converge more rapidly toward the targettemperature. As an example, if the target temperature is significantlygreater than the measured temperature, rather than starting with oneadditional LED at a low current value, two or more of the additionalLEDs can be driven at a current value greater than the initial, lowcurrent values to more rapidly increase the temperature of the LED unit.One of ordinary skill in the art would recognize many variations,modifications, and alternatives.

Thus, the temperature of LED unit 106 can be controlled using PIDcontroller 132 in conjunction with the operation of the various LEDs. Asdescribed above, in some embodiments, PID controller 132 drives the LEDsacross their full current range to implement the desired range of LEDunit temperatures.

It should be appreciated that the specific steps illustrated in FIG. 4provide a particular method of setting LED currents using a PIDcontroller according to an embodiment of the present invention. Othersequences of steps may also be performed according to alternativeembodiments. For example, alternative embodiments of the presentinvention may perform the steps outlined above in a different order.Moreover, the individual steps illustrated in FIG. 4 may includemultiple sub-steps that may be performed in various sequences asappropriate to the individual step. Furthermore, additional steps may beadded or removed depending on the particular applications. One ofordinary skill in the art would recognize many variations,modifications, and alternatives.

In order to control the temperature of LED unit 106, one or more of theLEDs in LED unit 106 are driven using drive current to generate heat inthe LED unit, thereby increasing or decreasing the temperature of theLED unit in a controllable manner. As an example, during the measurementof the characteristics of red LED (LED-R1), one or more of the other redLED (LED-R2), the green LEDs (LED-G1 and LED-G2) and/or the blue LEDs(LED-B1 and LED-B2) can be operated at given currents to generate adesired amount of heating of LED unit 106. Once the desired temperaturefor the LED unit is achieved, or a temperature within a threshold of thedesired temperature, the drive currents for each of the LEDs beingoperated, except for the LED being characterized, e.g., red LED(LED-R1), are set to zero and the LED being characterized is operated.Accordingly, only the red LED (LED-R1) will be illuminating LCoSprojector 104 during measurement of the spectral properties of the redLED at the desired temperature. The illumination of LCoS projector 104by the red LED can be maintained for the normal operating time (e.g.,˜2.7 ms) to reproduce the normal operating characteristics.

Accordingly, in the embodiment illustrated in FIG. 3, each light sourceis characterized independently while using the other light sources tofacilitate control of the optical package containing the light sources.

Although the various LEDs in LED unit 106 are utilized to enabletemperature control in the embodiment illustrated in FIG. 3, this is notrequired by the present invention and other implementations can beutilized to provide temperature control including a thermoelectriccooler, or the like.

Referring to FIG. 3, target LED currents may be provided by processingunit 130 to LED current controller 122, which generates LED currents foreach of the LEDs associated with each depth plane, for example, two setsof RGB currents (i_(R), i_(G), and i_(B)). As described herein, thecurrents can be generated concurrently and independently. Processingunit 130 may control the characterization process in use as well as theorder of characterization steps. Processing unit 130 may provide thetarget temperature to HD controller 132. Synchronization betweenspectrometer 214, LED current controller 122, and processing unit 130 isutilized to collect the spectral data in conjunction with the driving ofthe LEDs by LED current controller 122.

As illustrated in FIG. 3, the temperature of the thermistor in LED unit106 is provided to both HD controller 132 and processing unit 130. Thetemperature provided to PID controller 132 is utilized in the controlalgorithm used to control the temperature of LED unit 106B. Thetemperature provided to processing unit 130 is used to record thetemperate in conjunction with the spectral measurements at a given LEDcurrent.

Returning to the description of the calibration of the first red LED(LED-R1), the drive current is applied to the LED being characterized(i.e., i_(R)) and the chromaticity and the luminance (xyY) produced byeyepiece 102 are measured using spectrometer 214. During the measurementby the spectrometer, the temperature of LED unit 106 is also measured,for example, using the thermistor. The measured spectral properties(e.g., xyY) and the measured temperature are recorded by processing unit130 and/or calibration computer 202. Thus, as illustrated in FIG. 3, adatabase of chromaticity and luminance for each light source can beassembled as a function of LED unit temperature and drive current(i_(R)).

FIGS. 5 and 6 illustrate characterization of the other two LEDsassociated with a depth plane. As discussed in relation tocharacterization of LED-R1, one or more of the various LEDs are operatedin conjunction with PID controller 132 to control the temperature of LEDunit 106 in advance of collection of spectral measurements for the LEDbeing characterized. Similar procedures as those discussed in relationto the first depth plane can be utilized to characterize the three LEDsassociated with the second depth plane. One of ordinary skill in the artwould recognize many variations, modifications, and alternatives.

FIG. 5 illustrates an example characterization of LED-G1. Similar to thecharacterization of LED-R1, spectrometer 214 is positioned so as tomeasure the chromaticity and the luminance of the light output by LED-G1modulated by LCoS projector 104. During the characterization of LED-G1,controller 108 causes the current powering LED-G1, i_(G) of drivecurrent(s) 110, to iterate through a plurality of discrete currents andfor temperature 112 of LED unit 106 to vary across a range oftemperatures. Temperature control of the LED Unit can be achieved bycontrolling the currents of the non-characterized LEDs. For example, LEDcurrent controller 122, working in conjunction with PID controller 132may increase or decrease the current values for currents i_(R) and i_(B)so as to increase or decrease temperature 112 of LED unit 106. Duringthe characterization of LED-G1, synchronization between spectrometer 214and processing unit 130 (which is thereby synchronized with PIDcontroller 132 and LED current controller 122) is utilized such thatLCoS projector 104B reflects only light output from LED-G1 for eventualdelivery through eyepiece 102. As described herein, one or more of theLEDs (e.g., all the LEDs) can be driven at specific current levels togenerate the desired LED Unit temperature. Once the desired temperatureis achieved, the LED being characterized can be operated to illuminateLCoS projector 104 for a predetermined time period.

FIG. 6 illustrates an example characterization of LED-B1. Similar to thecharacterizations of LED-R1 and LED-G1, spectrometer 214 is positionedso as to measure the chromaticity and the luminance of the light outputby LED-B1 modulated by LCoS projector 104. During the characterizationof LED-B1, controller 108 causes the current powering LED-B1, i_(B) ofdrive current(s) 110, to iterate through a plurality of discretecurrents and for temperature 112 of LED unit 106 to vary across a rangeof temperatures. Temperature control of the LED Unit can be achieved bycontrolling the currents of the non-characterized LEDs. For example, LEDcurrent controller 122, working in conjunction with PID controller 132may increase or decrease the current values for currents i_(R) and i_(G)so as to increase or decrease temperature 112 of LED unit 106. Duringthe characterization of LED-B1, synchronization between spectrometer 214and processing unit 130 (which is thereby synchronized with PIDcontroller 132 and LED current controller 122) is utilized such thatLCoS projector 104 reflects only light output from LED-B1 for eventualdelivery through eyepiece 102. As described herein, one or more of theLEDs (e.g., all the LEDs) can be driven at specific current levels togenerate the desired LED Unit temperature. Once the desired temperatureis achieved, the LED being characterized can be operated to illuminatethe LCoS projector 104 for a predetermined time period.

FIG. 7 illustrates a method 700 for characterizing primary LEDs of anoptical system such as display device 100 according to an embodiment ofthe present invention. One or more steps of method 700 may be performedin a different order than that shown in FIG. 7, and not all steps needbe performed. In some embodiments, steps 702 to 708 may be performed foreach primary color LED. At step 702, xyY values are measured over arange of temperatures and currents to obtain a plurality of measurementpoints. Each measurement point of the plurality of measurement pointsincludes a current, a temperature, and an xyY value.

At step 704, the plurality of measurement points are converted from xyYspace to XYZ space. At step 706, the XYZ values areinterpolated/extrapolated over discrete temperatures to modify theplurality of measurement points. At step 708, the plurality ofmeasurement points are sorted by luminance at each temperature. In someembodiments, steps 710 and 712 may be performed for each targetluminance Y_(W) and for each temperature. At step 710, primaryluminances Y_(R), Y_(G), and Y_(B) are calculated using the chromaticityvalues for each primary color LED. As described more fully below inrelation to FIG. 8B, for the initial calculation of the primaryluminances, the chromaticity values for each of the LEDs measured at themedian current can be used. In some embodiments, method 700 is performedwith respect to a particular white point (e.g., D65).

At step 712, the XYZ values and the currents for each primary color areupdated (e.g., modified) by interpolating/extrapolating for each primarycolor target luminance. In some embodiments, steps 710 and 712 arerepeated until the XYZ values and/or the currents converge. Theresulting currents may be referred to as calculated currents. In otherembodiments, or in the same embodiments, steps 710 and 712 are repeateda predetermined number of times. Additional description related to thecalibration process illustrated in FIG. 7 is provided in relation toFIGS. 8A-8C. At step 714, the currents (and optionally the XYZ values)calculated during the last iteration of step 712 are stored in atwo-dimensional (2D) lookup table (LUT).

It should be appreciated that the specific steps illustrated in FIG. 7provide a particular method of characterizing primary LEDs of an opticalsystem according to another embodiment of the present invention. Othersequences of steps may also be performed according to alternativeembodiments. For example, alternative embodiments of the presentinvention may perform the steps outlined above in a different order.Moreover, the individual steps illustrated in FIG. 7 may includemultiple sub-steps that may be performed in various sequences asappropriate to the individual step. Furthermore, additional steps may beadded or removed depending on the particular applications. One ofordinary skill in the art would recognize many variations,modifications, and alternatives.

FIGS. 8A-8C illustrate a method 800 of characterizing primary color LEDsof an optical system according to an embodiment of the presentinvention. As discussed in relation to FIG. 3, the temperature of LEDunit 106 may be controlled using HD controller 108 and the chromaticityand luminance (xyY) will be measured for each LED at a given drivecurrent. These measurements will be repeated over a range oftemperatures and drive currents (step 802). Although only measurementsfor the red LED are illustrated in FIG. 8A, it will be appreciated thatsimilar databases can be assembled for each of the LEDs in the LED unit.The chromaticity and luminance measurements made in xyY space areconverted to absolute color space in the form of XYZ color space (step804).

During the measurement process, the measured LED unit temperature mayvary from the target temperature as illustrated by the variation intemperature for measurements at various currents from a singletemperature. Accordingly, interpolation/extrapolation is performed todetermine interpolated drive current values and interpolated XYZcoordinates for discrete LED unit temperatures (step 806), with thetemperature being used as the input in the interpolation calculation.Although linear interpolation is utilized in some embodiments, otherinterpolation techniques can be utilized in other embodiments. Thus,drive current values and XYZ values associated with discretetemperatures are provided as illustrated in FIG. 8A.

In order to assemble a database of drive current values and XYZ valuesassociated with discrete target luminances, the chromaticity andluminance data entries are sorted by luminance at each discretetemperature (step 808). The collection of xyY data over a range oftemperatures and drive currents, conversion to XYZ space,interpolation/extrapolation to align the current and XYZ data withdiscrete temperatures and sorting by luminance at each temperature arealso performed for the other primary color LEDs.

Referring to FIG. 8B, given a target luminance, the desired white point(e.g., D65), and the chromaticities of the primary colors, the requiredprimary color luminances are calculated (step 810). In other words, itis possible to solve for the red, green, and blue luminance ratios givena desired output chromaticity and the measured red, green, bluechromaticities in xyY space. Referring to FIG. 8B, for a given targetluminance of 1, a desired white point of x=0.3127 and y=0.329 (i.e.,D65), and chromaticities of x=0.6972 and y=0.3022 for red; x=0.154 andy=0.7681 for green; and x=0.1484 and y=0.029 for blue, required colorluminances of Y=0.272 for red, Y=0.692 for green, and Y=0.036 for bluecan be computed. The required color luminance for each primary colordivided by the output luminance can be referred to as a luminance ratio(LR). In the example illustrated in FIG. 8B, since the output luminanceis 1, the LRs are the required color luminances. If the output luminancewas 100 nits, then, for this example, the required color luminance wouldbe 27.2 nits for red, 69.2 nits for green, and 3.6 nits for blue.

As a starting point for the iteration, the chromaticity measured foreach primary color at the median current can be utilized. Additionally,the measured chromaticities at the LED unit temperature can be utilizedto provide more accurate initial primary color chromaticities. However,during actual operation, for example, as the temperature of the LED unitchanges as a result of ambient temperature and/or as the temperature ofthe LED unit changes as a result of temperature variation of the LEDs inthe LED unit (for example, as the drive current provided to the LEDsvaries), the chromaticity of the red, green, and blue LEDs can vary fromthis median current chromaticity. In addition to the median current,other nominal values for LED chromaticity can be used.

Given the initially computed required primary color luminances for thethree primary colors, these values will be used to index the orderedlist sorted by luminance as illustrated in step 808 of FIG. 8A. Usingthe luminance values, the drive currents will beinterpolated/extrapolated to obtain the drive currents associated withthe computed luminance values. As illustrated in FIG. 8A, the array ofluminance values as a function of drive current for each color at eachtemperature can be utilized to determine the drive current associatedwith each computed luminance value. In some implementations, a linearinterpolation between drive currents is utilized to determine theinterpolated drive current, which will typically vary from the mediancurrent used to provide the initial chromaticity.

As an example, if, at a temperature of 300° K, a drive current for thered LED of 100 mA produces a luminance of 20 nits and a drive current of200 mA produces a luminance of 30 nits, then a linear interpolation canbe used to determine that, in order to produce the computed redluminance of 27.2 nits, a drive current of 172 mA will produce thedesired 27.2 nits of luminance.

Additionally, the XYZ color space values will beinterpolated/extrapolated using the computed luminance values. Using adata set including the primary color luminance as a function of XYZcoordinates, in a manner similar to the LED drive current interpolation,the XYZ coordinates can be interpolated that will produce the desiredluminance.

Given the interpolated chromaticity values (based on the interpolatedXYZ values), an interactive loop will be utilized to compute updatedprimary color luminances based on the desired white point, the targetluminance, and the interpolated chromaticity values.

In some embodiments, the difference between each of the primary colorchromaticities is used for comparison with a threshold, whereas in otherembodiments, the difference in the initial chromaticity and the updatedchromaticity for the color being calibrated (e.g., red in this example)is utilized. In other embodiments, an average difference is computed andutilized. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives. Although a difference ininitial and updated chromaticities is utilized in the embodimentillustrated in FIG. 8B, this is not required by the present inventionand other methods, for example, iterating a predetermined number oftimes (e.g., 5, 10, 50, 100, 500, 1000, or 5000 iterations) can beutilized.

As illustrated in FIG. 8B, the method of forming a luminance axis of atwo-dimensional LUT includes, at step 810, calculating required primarycolor luminances using a desired white point, a target luminance, andinitial chromaticities of the primary colors. The initial chromaticitiescan be set as the chromaticities measured at a median drive currentlevel.

The method also includes, for each primary color's computed luminance,at step 812, interpolating the drive current that is associated witheach primary color luminance and, at step 814, interpolating the XYZcoordinate that is associated with each primary color luminance. Themethod further includes, at step 816, determining updated chromaticitiesbased on the interpolated XYZ coordinates. If it is determined that thedifference in one or more or an average of the interpolatedchromaticities and the primary color chromaticities is greater than athreshold at step 818, then the method iterates, at step 820, using thenew interpolated chromaticities for the primary color chromaticities.The iteration process can also be based on a predetermined number ofiterations. Accordingly, after the iterative interpolation process, foreach LED, drive currents and XYZ coordinates are aligned with discreteluminance values. The 2D LUT illustrated in FIG. 8C is thus generatedbased on the interpolated drive currents and XYZ coordinates fordiscrete LED unit temperatures and discrete luminance values and can bestored for use by the feed forward control system described herein.

Referring back to FIG. 8A, the luminance at each temperature was sortedto provide the sorted luminance values, as shown at step 808. Theprocess illustrated in FIG. 8B will be repeated at each discretetemperature, producing the 2D LUT defining drive currents as a functionof LED unit temperature and target luminance as shown in FIG. 8C. Thus,if there are M target luminances and N discrete temperatures, theiterative process illustrated in FIG. 8B will be performed M×N times.

Although only drive currents are illustrated in FIG. 8C, the 2D LUT canalso store chromaticity and luminance information such as the XYZcoordinates as a function of temperature and luminance in addition tothe drive currents (step 822). For example, the XYZ coordinates can beused to reduce the color gamut to a standard format. Thus, in FIG. 8C,the entries in the 2D LUT can be expanded from {i_(R), i_(G), i_(B)} to{i_(R), i_(G), i_(B), X_(R), X_(G), X_(B), Y_(R), Y_(G), Y_(B), Z_(R),Z_(G), Z_(B)}. Thus, in addition to indexed drive currents for use inthe feed forward control system, other indexed values can be stored inthe 2D LUT for use by various system elements.

Although embodiments are described in terms of achieving D65 as a whitepoint, the present invention is not limited to this white point. Inorder to provide alternative white points, embodiments of the presentinvention can be utilized to provide alternative 2D LUTs that areassociated with alternative white points. As illustrated in FIG. 8B, thetarget chominance values are inputs (given variables) to the computationof the primary color luminance values and modification of these inputswill result in 2D LUTs with modified entries appropriate to thealternative white points. In other embodiments, the implementation toprovide alternative white points can be implemented at the level ofcontroller 108.

It should be appreciated that the specific steps illustrated in FIG. 8Bprovide a particular method of determining drive currents and XYZcoordinates for each target luminance and each target temperatureaccording to another embodiment of the present invention. Othersequences of steps may also be performed according to alternativeembodiments. For example, alternative embodiments of the presentinvention may perform the steps outlined above in a different order.Moreover, the individual steps illustrated in FIG. 8B may includemultiple sub-steps that may be performed in various sequences asappropriate to the individual step. Furthermore, additional steps may beadded or removed depending on the particular applications. One ofordinary skill in the art would recognize many variations,modifications, and alternatives.

In relation to the calculation of the primary color luminance ratios, tosolve for the luminance ratios, equations that calculate the outputchromaticity based on the contributing chromaticities and luminance areused. Luminances can be added in xyY or XYZ space to get the outputluminance. However, as xyY is a non-linear space, chromaticities areonly added when expressed in XYZ space.

To convert from xyY space to XYZ space, the following equations areused:

$X = {{\frac{Y}{y}x\mspace{20mu} Y} = {{Y\mspace{20mu} Z} = {\frac{Y}{y}\left( {1 - x - y} \right)}}}$

Conversion from XYZ space to xyY space is performed as:

$x = {{\frac{X}{X + Y + Z}\mspace{20mu} y} = {{\frac{Y}{X + Y + Z}\mspace{20mu} Y} = Y}}$

In order to calculate the primary color luminance ratios, the XYZsummation equations can be rewritten with xyY primary coordinatessubstituted as:

$\mspace{79mu} {{{\left( \frac{x_{R}}{y_{R}} \right)Y_{R}} + {\left( \frac{x_{G}}{y_{G}} \right)Y_{G}} + {\left( \frac{x_{B}}{y_{B}} \right)Y_{B}}} = {\left( \frac{x_{OUT}}{y_{OUT}} \right)Y_{OUT}}}$$\mspace{20mu} {{Y_{R} + Y_{G} + Y_{B}} = {{{{Y_{OUT}\left( \frac{1 - x_{R} - y_{R}}{y_{R}} \right)}Y_{R}} + {\left( \frac{1 - x_{G} - y_{G}}{y_{G}} \right)Y_{G}} + {\left( \frac{1 - x_{B} - y_{B}}{y_{B}} \right)Y_{B}}} = {\left( \frac{1 - x_{OUT} - y_{OUT}}{y_{OUT}} \right)Y_{OUT}}}}$

This equation can be rewritten in matrix form as:

$\begin{bmatrix}{\left( \frac{x_{OUT}}{y_{OUT}} \right)Y_{OUT}} \\Y_{OUT} \\{\left( \frac{1 - x_{OUT} - y_{OUT}}{y_{OUT}} \right)Y_{OUT}}\end{bmatrix} = {\quad{\begin{bmatrix}\left( \frac{x_{R}}{y_{R}} \right) & \left( \frac{x_{G}}{y_{G}} \right) & \left( \frac{x_{B}}{y_{B}} \right) \\1 & 1 & 1 \\\left( \frac{1 - x_{R} - y_{R}}{y_{R}} \right) & \left( \frac{1 - x_{G} - y_{G}}{y_{G}} \right) & \left( \frac{1 - x_{B} - y_{B}}{y_{B}} \right)\end{bmatrix}\begin{bmatrix}Y_{R} \\Y_{G} \\Y_{B}\end{bmatrix}}}$

Rearranging to solve for Y_(RGB) yields:

$\begin{bmatrix}Y_{R} \\Y_{G} \\Y_{B}\end{bmatrix} = {\quad{\begin{bmatrix}\left( \frac{x_{R}}{y_{R}} \right) & \left( \frac{x_{G}}{y_{G}} \right) & \left( \frac{x_{B}}{y_{B}} \right) \\1 & 1 & 1 \\\left( \frac{1 - x_{R} - y_{R}}{y_{R}} \right) & \left( \frac{1 - x_{G} - y_{G}}{y_{G}} \right) & \left( \frac{1 - x_{B} - y_{B}}{y_{B}} \right)\end{bmatrix}^{- 1}{\quad\begin{bmatrix}{\left( \frac{x_{OUT}}{y_{OUT}} \right)Y_{OUT}} \\Y_{OUT} \\{\left( \frac{1 - x_{OUT} - y_{OUT}}{y_{OUT}} \right)Y_{OUT}}\end{bmatrix}}}}$

To compute the luminance ratios, both sides can be divided by Y_(OUT) toyield:

$\begin{bmatrix}{LR}_{R} \\{LR}_{G} \\{LR}_{B}\end{bmatrix} = {\begin{bmatrix}\frac{Y_{R}}{Y_{OUT}} \\\frac{Y_{G}}{Y_{OUT}} \\\frac{Y_{B}}{Y_{OUT}}\end{bmatrix} = {\quad{\begin{bmatrix}\left( \frac{x_{R}}{y_{R}} \right) & \left( \frac{x_{G}}{y_{G}} \right) & \left( \frac{x_{B}}{y_{B}} \right) \\1 & 1 & 1 \\\left( \frac{1 - x_{R} - y_{R}}{y_{R}} \right) & \left( \frac{1 - x_{G} - y_{G}}{y_{G}} \right) & \left( \frac{1 - x_{B} - y_{B}}{y_{B}} \right)\end{bmatrix}^{- 1}{\quad\left\lbrack \begin{matrix}\left( \frac{x_{OUT}}{y_{OUT}} \right) \\1 \\\left( \frac{1 - x_{OUT} - y_{OUT}}{y_{OUT}} \right)\end{matrix} \right\rbrack}}}}$

In a similar fashion, the luminance ratios can be determined forprimaries and the desired output given in XYZ space. To solve for theluminances (i.e., luminance ratios) in the above example, thechromiticities may be converted to XYZ space because xyY is a non-linearspace and XYZ is a linear space. In some embodiments, conversion fromxyY space to XYZ space may be performed using the following equations:X=Y/y*x, Y=Y, and Z=Y/y*(1−x−y). Conversely, conversion from XYZ spaceto xyY space may be performed using the following equations:x=X/(X+Y+Z), y=Y/(X+Y+Z), and Y=Y. After converting to XYZ space, theluminance ratios can be calculated using the following equation:

$\begin{bmatrix}{LR}_{R} \\{LR}_{G} \\{LR}_{B}\end{bmatrix} = {\begin{bmatrix}\frac{Y_{R}}{Y_{OUT}} \\\frac{Y_{G}}{Y_{OUT}} \\\frac{Y_{B}}{Y_{OUT}}\end{bmatrix} = {\begin{bmatrix}\left( \frac{X_{R}}{Y_{R}} \right) & \left( \frac{X_{G}}{Y_{G}} \right) & \left( \frac{X_{B}}{Y_{B}} \right) \\1 & 1 & 1 \\\left( \frac{X_{R}}{Y_{R}} \right) & \left( \frac{X_{G}}{Y_{G}} \right) & \left( \frac{X_{B}}{Y_{B}} \right)\end{bmatrix}^{- 1}\begin{bmatrix}\frac{X_{OUT}}{Y_{OUT}} \\1 \\\frac{Z_{OUT}}{Y_{OUT}}\end{bmatrix}}}$

In the previous calibration, it is found the luminance of each LED(Y_(sRGB_*_TO_W)) to hit the target sRGB white point (0.3127, 0.3290) ata given brightness. The native LED gamut is reduced to the sRGB gamut,while maintaining the same white point.

The same function used before may be used to calculate the LEDs'luminance ratios needed for the desired white point to calculate theLEDs' luminance ratios for each sRGB primary.

For sRGB red:

{LR_(R_TO_R_NATIVE), LR_(G_TO_R_NATIVE), LR_(B_TO_R_NATIVE)}GET_LR(out_xy=0.6400, 0.3300, native_xy_rgb)

For sRGB green:

{LR_(R_TO_G_NATIVE), LR_(G_TO_G_NATIVE),LR_(B_TO_G_NATIVE)}=GET_LR(out_xy=0.3000, 0.6000, native_xy_rgb)

For sRGB blue:

{LR_(R_TO_B_NATIVE) , LR _(G_TO_B_NATIVE),LR_(B_TO_B_NATIVE)}=GET_LR(out_xy=0.1500, 0.0600, native_xy_rgb)

Since the chromaticity of the primary colors are changed, the luminancerations for sRGB White can be computed as:

{LR_(R_TO_W), LR_(G_TO_W), LR_(B_TO_W)}=GET_LR(out_xy={0.3127, 0.3290,sRGB_xy={0.6400, 0.3300}, {0.3000, 0.6000}, {0.1500, 0.0600})

The conversion of a pixel's value in RGB space to LEDs' luminance valuecan be expressed as:

$\begin{bmatrix}Y_{R\; \_ \; {OUT}} \\Y_{G\; \_ \; {OUT}} \\Y_{B\; \_ \; {OUT}}\end{bmatrix} = {{\begin{bmatrix}Y_{R\; \_ \; {NATIVE}} & 0 & 0 \\0 & Y_{G\; \_ \; {NATIVE}} & 0 \\0 & 0 & Y_{B\; \_ \; {NATIVE}}\end{bmatrix}\begin{bmatrix}k_{00} & k_{01} & k_{02} \\k_{10} & k_{11} & k_{12} \\k_{20} & k_{21} & k_{22}\end{bmatrix}}\begin{bmatrix}R_{in} \\G_{in} \\B_{in}\end{bmatrix}}$

where the 3×3 of constants k_(ij) is the color space conversion (CSC)matrix that we will use to reduce the gamut.

The CSC matrix can be solved such that the following conditions are met:

When RGB_(IN)=[1 0 0 ]^(T),

Y _(OUT) =[Y _(W_NATIVE)*LR_(R_TO_W)*LR_(R_TO_R_NATIVE) , Y_(W_NATIVE)*LR_(R_TO_W)*LR_(G_TO_R_NATIVE) , Y_(W_NATIVE)*LR_(R_TO_W)*LR_(B_TO_R_NATIVE)]^(T)

When RGB_(IN)=[0 1 0]^(T),

Y _(OUT) =[Y _(W_NATIVE)*LR_(G_TO_W)*LR_(R_TO_G_NATIVE) , Y_(W_NATIVE)*LR_(G_TO_W)*LR_(G_TO_G_NATIVE) , Y_(W_NATIVE)*LR_(G_TO_W)*LR_(B_TO_G_NATIVE)]^(T)

When RGB_(IN)=[0 0 1]^(T),

Y _(OUT) =[Y _(W_NATIVE)*LR_(B_TO_W)*LR_(R_TO_B_NATIVE) , Y_(W_NATIVE)*LR_(B_TO_W)*LR_(G_TO_B_NATIVE) , Y_(W_NATIVE)*LR_(B_TO_W)*LR_(B_TO_B_NATIVE)]^(T)

By this construction, it is guaranteed that the following condition isalso met:

When RGB_(IN)=[1 1 1]^(T),

Y_(OUT)=[Y_(R_NATIVE), Y_(G_NATIVE), Y_(B_NATIVE)]^(T).

Substituting RGB_(IN)=[1 0 0]^(T) and Y _(OUT) =[Y_(W_NATIVE)*LR_(R_TO_W)*LR_(R_TO_R_NATIVE) , Y_(W_NATIVE)*LR_(R_TO_W)*LR_(G_TO_R_NATIVE) , Y_(W_NATIVE)*LR_(R_TO_W)*LR_(B_TO_R_NATIVE)]^(T)

${\frac{Y_{W\; \_ \; {NATIVE}} \times {LR}_{R\; \_ \; {TO}\; \_ \; W} \times {LR}_{R\; \_ \; {TO}\; \_ \; R\; \_ \; {NATIVE}}}{Y_{R\; \_ \; {NATIVE}}} = k_{00}}{\frac{Y_{W\; \_ \; {NATIVE}} \times {LR}_{R\; \_ \; {TO}\; \_ \; W} \times {LR}_{G\; \_ \; {TO}\; \_ \; R\; \_ \; {NATIVE}}}{Y_{G\; \_ \; {NATIVE}}} = k_{10}}{\frac{Y_{W\; \_ \; {NATIVE}} \times {LR}_{R\; \_ \; {TO}\; \_ \; W} \times {LR}_{B\; \_ \; {TO}\; \_ \; R\; \_ \; {NATIVE}}}{Y_{B\; \_ \; {NATIVE}}} = k_{20}}$Substituting RGB_(IN)=[0 1 0]^(T) and Y _(OUT) =[Y_(W_NATIVE)×LR_(G_TO_W)*LR_(R_TO_G_NATIVE) , Y_(W_NATIVE)×LR_(G_TO_W)*LR_(G_TO_G_NATIVE),Y_(W_NATIVE)×Y_(G_TO_W)*LR_(B_TO_G_NATIVE]) ^(T)

${\frac{Y_{G\; \_ \; {NATIVE}} \times {LR}_{R\; \_ \; {TO}\; \_ \; G\; \_ \; {NATIVE}}}{Y_{R\; \_ \; {NATIVE}}} = k_{01}}{\frac{Y_{G\; \_ \; {NATIVE}} \times {LR}_{G\; \_ \; {TO}\; \_ \; G\; \_ \; {NATIVE}}}{Y_{G\; \_ \; {NATIVE}}} = k_{11}}{\frac{Y_{G\; \_ \; {NATIVE}} \times {LR}_{B\; \_ \; {TO}\; \_ \; G\; \_ \; {NATIVE}}}{Y_{B\; \_ \; {NATIVE}}} = k_{21}}$Substituting RGB_(IN)=[0 0 1]^(T) , Y _(OUT) =[Y_(W_NATIVE)×LR_(B_TO_W)*LR_(R_TO_B_NATIVE) , Y_(W_NATIVE)×LR_(B_TO_W)*LR_(G_TO_B_NATIVE) , Y_(W_NATIVE)×LR_(B_TO_W)*LR_(B_TO_B_NATIVE)]^(T)

${{\frac{Y_{B\; \_ \; {NATIVE}} \times {LR}_{R\; \_ \; {TO}\; \_ \; B\; \_ \; {NATIVE}}}{Y_{R\; \_ \; {NATIVE}}} = k_{02}}{\frac{Y_{B\; \_ \; {NATIVE}} \times {LR}_{G\; \_ \; {TO}\; \_ \; B\; \_ \; {NATIVE}}}{Y_{G\; \_ \; {NATIVE}}} = k_{12}}{\frac{Y_{B\; \_ \; {NATIVE}} \times {LR}_{B\; \_ \; {TO}\; \_ \; B\; \_ \; {NATIVE}}}{Y_{B\; \_ \; {NATIVE}}} = k_{22}}}.$

FIG. 9 illustrates a operation of a first set of primary LEDs of displaydevice 100 that have been previously characterized to generate 2D LUT910. As illustrated in FIG. 8C, the 2D LUT provides the drive currentsfor each of the LEDs in the first set of primary LEDs. When displaydevice 100 is in actual use by a user, the first set of primary LEDs areoperated by a process that includes measuring temperature 112, receivinga target luminance, accessing 2D LUT 910 to determine currents i_(R),i_(G), and i_(B), and driving the LEDs with the determined currents. Abilinear interpolation can be utilized in conjunction with the 2D LUT todetermine drive currents as a function of temperature and targetluminance for arbitrary temperatures and target luminances. Asillustrated in FIG. 9, the operating temperature 112 of the LED unit 106is measured using the thermistor and provided to feed forward calculator120 and used in determining the drive current for the various LEDs.

Referring to FIG. 9, a target chromaticity (e.g., D65) and luminance(e.g., 100 nits) are received at the feed forward calculator 120 fromprocessing unit 130. The temperature of LED unit 106 is received at feedforward calculator 120 from the thermistor. Using the 2D LUT thatdefines drive currents as a function of temperature and luminance, thetarget LED current signals (e.g., digital signals) are generated by feedforward calculator 120 and provided to LED current controller 122.Although only three currents are illustrated in FIG. 9, it will beappreciated that for a dual depth plane implementation with twoeyepieces (right and left), 12 target LED current signals will begenerated. LED current controller 122 then generates analog drivecurrents that are used to drive the corresponding LED. As illustrated inFIG. 9, three LEDs are driven (LED-R1, LED-G1, and LED-B1) to provideinputs to one depth plane of eyepiece 102 after reflection from LCoSprojector 104. In order to generate a uniform white light display, thepixels of the LCoS projector 104 will be set to a uniform (i.e.,maximum) reflectivity to reflect the light generated during thesequential operation of the primary color LEDs.

As the target luminance specified by processing unit 130 or thetemperature of the LED unit 106 changes, feed forward calculator 120will update the target LED currents based on these changes, therebyproviding output from the eyepiece 102 that is characterized by aconsistent white point at the target luminance. In some embodiments,feed forward calculator 120 updates the target LED currents at apredetermined periodic rate (e.g., every frame or a periodicity of ˜83ms).

FIG. 10 illustrates a operation of a second set of primary LEDs ofdisplay device 100 that have been previously characterized to generate2D LUT 1010. When display device 100 is in actual use by a user, thesecond set of primary LEDs are operated by a process that includesmeasuring temperature 112, receiving a target luminance, accessing 2DLUT 1010 to determine currents i_(R), i_(G), and i_(B), and driving theLEDs with the determined currents. As illustrated in FIG. 10, theoperating temperature 112 of LED unit 106 is measured using thethermistor and provided to the feed forward calculator 120 and used indetermining the drive current for the various LEDs, i.e., LED-R2,LED-G2, and LED-B2.

FIG. 11 illustrates a method 1100 for calibrating a display device, suchas display device 100. In some embodiments, method 1100 may be performedsubsequent to performance of methods 700 or 800. One or more steps ofmethod 1100 may be performed in a different order than that shown inFIG. 11, and not all steps need be performed. At step 1102, a targetluminance Y_(W) is received. The target luminance Y_(W) may correspondto a particular white point, for example, D65. At step 1104, thetemperature of the LED unit is measured. At step 1106, one or more 2Dlookup tables are accessed to determine currents i_(R), i_(G), andi_(B), which can be subsequently used to drive the primary color LEDs.

In step 1106, bi-linear interpolation can be used to determine the drivecurrent for each LED, i.e., i_(R), i_(G), and i_(B), as well as thechromaticity and luminance (i.e., x, y, and Y) for each of the LEDs.Accordingly, some embodiments determine 12 parameters: {i_(R), i_(G),i_(B), x_(R), x_(G), x_(B), y_(R), y_(G), y_(B), Y_(R), Y_(G), Y_(B)}.To perform the bi-linear interpolation, the discrete LED unittemperatures and luminances used for the 2D look up tables are accessedand the neighboring indexes for the LED unit's temperature and thetarget luminance are found. For each of the 12 parameters, therespective 2D LUT is accessed, the four neighboring entries are found,and bi-linear interpolation is performed. At step 1108, the primarycolor LEDs are driven with the determined currents.

It should be appreciated that the specific steps illustrated in FIG. 11provide a particular method of calibrating a display device according toanother embodiment of the present invention. Other sequences of stepsmay also be performed according to alternative embodiments. For example,alternative embodiments of the present invention may perform the stepsoutlined above in a different order. Moreover, the individual stepsillustrated in FIG. 11 may include multiple sub-steps that may beperformed in various sequences as appropriate to the individual step.Furthermore, additional steps may be added or removed depending on theparticular applications. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

FIG. 12 illustrates a block diagram of a feed forward controller 1200according to an embodiment of the present invention. Referring to FIG.12, the inputs to feed forward controller 1200 are the targetchromaticity and luminance in order to create a given virtual image(white point) and a disturbance, which is the measurement of thetemperature of the LED package, also referred to as an LED unit. In FIG.12, the LED package is referenced as CPC 1220 since the LED package canutilize CPCs. The thermal mass of the LED package can be used tofacilitate temperature control as described more fully herein. Asillustrated in FIG. 12, a thermistor 1210 is utilized to provide thetemperature of CPC 1220 to feed forward calculator 120 although othertemperature measurement devices can be utilized.

In certain ambient conditions, the temperature of the LED package canchange during the time period during which the spectral characterizationof the particular LED under test is being characterized. In this case,an average temperature during the test period can be computed.

As illustrated in FIG. 12, feed forward calculator 120 is able toprovide RGB color points (e.g., in xyY color space) as an output. Asdescribed in relation to FIG. 8C, the 2D LUT can include not only drivecurrent values, but chromaticity and luminance information for the LEDsat the drive currents of interest. Thus, this chromaticity and luminanceinformation can be output for use by other system elements, for example,for conversion between color gamuts, for example, from a native colorgamut to the sRGB color gamut.

The summation of the light emitted by the three LEDs occurs in the eyeof the viewer in implementations in which field sequential displaysystems are utilized.

The following discussion illustrates one or more steps of the colorcalibration process as described herein. As discussed herein, the drivecurrents applied to the LEDs to achieve a particular white point (e.g.,D65) may vary as the determined chromaticities for the red, green, andblue LEDs change as a function of temperature and drive current. Forexample, at a first time, a particular white point having a chromaticitycoordinate of x=0.3127, y=0.329 may be generated according to thefollowing table:

The calculated luminances for the red, green, and blue LEDs are listedin the bottom row and sum to the target luminance of the particularwhite point of 1.

Continuing with the above example, at a second time, the chromaticitycoordinates for the red, green, and blue LEDs have changed (due to,e.g., a change in temperature) and the same white point having achromaticity coordinate of x=0.3127, y=0.329 may be generated accordingto the following table:

The calculated luminances for the red, green, and blue LEDs are listedin the bottom row and again sum to the target luminance of theparticular white point of 1.

Continuing with the above example, at a second time, the chromaticitycoordinates for the red, green, and blue LEDs have changed (due to,e.g., a change in temperature) and the same white point having achromaticity coordinate of x=0.3127, y=0.329 may be generated accordingto the following table:

The calculated luminances for the red, green, and blue LEDs are listedin the bottom row and again sum to the target luminance of theparticular white point of 1.

It should be appreciated that the specific steps illustrated in thefigures related to the methods described herein illustrate steps thatprovide the particular methods described herein. Other sequences ofsteps may also be performed according to alternative embodiments. Forexample, alternative embodiments of the present invention may performthe steps outlined above in a different order. Moreover, the individualsteps illustrated in the figures related to the methods described hereinmay include multiple sub-steps that may be performed in varioussequences as appropriate to the individual step. Furthermore, additionalsteps may be added or removed depending on the particular applications.One of ordinary skill in the art would recognize many variations,modifications, and alternatives.

FIG. 13 illustrates a simplified computer system 1300 according to anembodiment described herein. Computer system 1200 as illustrated in FIG.12 may be incorporated into devices such as display device 100 andcalibration computer 202 as described herein. FIG. 13 provides aschematic illustration of one embodiment of computer system 1300 thatcan perform some or all of the steps of the methods provided by variousembodiments. It should be noted that FIG. 13 is meant only to provide ageneralized illustration of various components, any or all of which maybe utilized as appropriate. FIG. 13, therefore, broadly illustrates howindividual system elements may be implemented in a relatively separatedor relatively more integrated manner.

Computer system 1300 is shown comprising hardware elements that can beelectrically coupled via a bus 1305, or may otherwise be incommunication, as appropriate. The hardware elements may include one ormore processors 1310, including without limitation one or moregeneral-purpose processors and/or one or more special-purpose processorssuch as digital signal processing chips, graphics accelerationprocessors, and/or the like; one or more input devices 1315, which caninclude without limitation a mouse, a keyboard, a camera, and/or thelike; and one or more output devices 1320, which can include withoutlimitation a display device, a printer, and/or the like.

Computer system 1300 may further include and/or be in communication withone or more non-transitory storage devices 1325, which can comprise,without limitation, local and/or network accessible storage, and/or caninclude, without limitation, a disk drive, a drive array, an opticalstorage device, a solid-state storage device, such as a random accessmemory (“RAM”), and/or a read-only memory (“ROM”), which can beprogrammable, flash-updateable, and/or the like. Such storage devicesmay be configured to implement any appropriate data stores, includingwithout limitation, various file systems, database structures, and/orthe like.

Computer system 1300 might also include a communications subsystem 1319,which can include without limitation a modem, a network card (wirelessor wired), an infrared communication device, a wireless communicationdevice, and/or a chipset such as a Bluetooth™ device, an 802.11 device,a WiFi device, a WiMax device, cellular communication facilities, etc.,and/or the like. The communications subsystem 1319 may include one ormore input and/or output communication interfaces to permit data to beexchanged with a network such as the network described below to name oneexample, other computer systems, television, and/or any other devicesdescribed herein. Depending on the desired functionality and/or otherimplementation concerns, a portable electronic device or similar devicemay communicate image and/or other information via the communicationssubsystem 1319. In other embodiments, a portable electronic device, e.g.the first electronic device, may be incorporated into computer system1300, e.g., an electronic device as an input device 1315. In someembodiments, computer system 1300 will further comprise a working memory1335, which can include a RAM or ROM device, as described above.

Computer system 1300 also can include software elements, shown as beingcurrently located within the working memory 1335, including an operatingsystem 1340, device drivers, executable libraries, and/or other code,such as one or more application programs 1345, which may comprisecomputer programs provided by various embodiments, and/or may bedesigned to implement methods, and/or configure systems, provided byother embodiments, as described herein. Merely by way of example, one ormore procedures described with respect to the methods discussed above,might be implemented as code and/or instructions executable by acomputer and/or a processor within a computer; in an aspect, then, suchcode and/or instructions can be used to configure and/or adapt a generalpurpose computer or other device to perform one or more operations inaccordance with the described methods.

A set of these instructions and/or code may be stored on anon-transitory computer-readable storage medium, such as the storagedevice(s) 1325 described above. In some cases, the storage medium mightbe incorporated within a computer system, such as computer system 1300.In other embodiments, the storage medium might be separate from acomputer system e.g., a removable medium, such as a compact disc, and/orprovided in an installation package, such that the storage medium can beused to program, configure, and/or adapt a general purpose computer withthe instructions/code stored thereon. These instructions might take theform of executable code, which is executable by computer system 1300and/or might take the form of source and/or installable code, which,upon compilation and/or installation on computer system 1300 e.g., usingany of a variety of generally available compilers, installationprograms, compression/decompression utilities, etc., then takes the formof executable code.

It will be apparent to those skilled in the art that substantialvariations may be made in accordance with specific requirements. Forexample, customized hardware might also be used, and/or particularelements might be implemented in hardware, software including portablesoftware, such as applets, etc., or both. Further, connection to othercomputing devices such as network input/output devices may be employed.

As mentioned above, in one aspect, some embodiments may employ acomputer system such as computer system 1300 to perform methods inaccordance with various embodiments of the technology. According to aset of embodiments, some or all of the procedures of such methods areperformed by computer system 1300 in response to processor 1310executing one or more sequences of one or more instructions, which mightbe incorporated into the operating system 1340 and/or other code, suchas an application program 1345, contained in the working memory 1335.Such instructions may be read into the working memory 1335 from anothercomputer-readable medium, such as one or more of the storage device(s)1325. Merely by way of example, execution of the sequences ofinstructions contained in the working memory 1335 might cause theprocessor(s) 1310 to perform one or more procedures of the methodsdescribed herein. Additionally or alternatively, portions of the methodsdescribed herein may be executed through specialized hardware.

The terms “machine-readable medium” and “computer-readable medium,” asused herein, refer to any medium that participates in providing datathat causes a machine to operate in a specific fashion. In an embodimentimplemented using computer system 1300, various computer-readable mediamight be involved in providing instructions/code to processor(s) 1310for execution and/or might be used to store and/or carry suchinstructions/code. In many implementations, a computer-readable mediumis a physical and/or tangible storage medium. Such a medium may take theform of a non-volatile media or volatile media. Non-volatile mediainclude, for example, optical and/or magnetic disks, such as the storagedevice(s) 1325. Volatile media include, without limitation, dynamicmemory, such as the working memory 1335.

Common forms of physical and/or tangible computer-readable mediainclude, for example, a floppy disk, a flexible disk, hard disk,magnetic tape, or any other magnetic medium, a CD-ROM, any other opticalmedium, punchcards, papertape, any other physical medium with patternsof holes, a RAM, a PROM, EPROM, a FLASH-EPROM, any other memory chip orcartridge, or any other medium from which a computer can readinstructions and/or code.

Various forms of computer-readable media may be involved in carrying oneor more sequences of one or more instructions to the processor(s) 1310for execution. Merely by way of example, the instructions may initiallybe carried on a magnetic disk and/or optical disc of a remote computer.A remote computer might load the instructions into its dynamic memoryand send the instructions as signals over a transmission medium to bereceived and/or executed by computer system 1300.

The communications subsystem 1319 and/or components thereof generallywill receive signals, and the bus 1305 then might carry the signalsand/or the data, instructions, etc. carried by the signals to theworking memory 1335, from which the processor(s) 1310 retrieves andexecutes the instructions. The instructions received by the workingmemory 1335 may optionally be stored on a non-transitory storage device1325 either before or after execution by the processor(s) 1310.

The methods, systems, and devices discussed above are examples. Variousconfigurations may omit, substitute, or add various procedures orcomponents as appropriate. For instance, in alternative configurations,the methods may be performed in an order different from that described,and/or various stages may be added, omitted, and/or combined. Also,features described with respect to certain configurations may becombined in various other configurations. Different aspects and elementsof the configurations may be combined in a similar manner. Also,technology evolves and, thus, many of the elements are examples and donot limit the scope of the disclosure or claims.

Specific details are given in the description to provide a thoroughunderstanding of exemplary configurations including implementations.However, configurations may be practiced without these specific details.For example, well-known circuits, processes, algorithms, structures, andtechniques have been shown without unnecessary detail in order to avoidobscuring the configurations. This description provides exampleconfigurations only, and does not limit the scope, applicability, orconfigurations of the claims. Rather, the preceding description of theconfigurations will provide those skilled in the art with an enablingdescription for implementing described techniques. Various changes maybe made in the function and arrangement of elements without departingfrom the spirit or scope of the disclosure.

Also, configurations may be described as a process which is depicted asa schematic flowchart or block diagram. Although each may describe theoperations as a sequential process, many of the operations can beperformed in parallel or concurrently. In addition, the order of theoperations may be rearranged. A process may have additional steps notincluded in the figure. Furthermore, examples of the methods may beimplemented by hardware, software, firmware, middleware, microcode,hardware description languages, or any combination thereof. Whenimplemented in software, firmware, middleware, or microcode, the programcode or code segments to perform the necessary tasks may be stored in anon-transitory computer-readable medium such as a storage medium.Processors may perform the described tasks.

Having described several example configurations, various modifications,alternative constructions, and equivalents may be used without departingfrom the spirit of the disclosure. For example, the above elements maybe components of a larger system, wherein other rules may takeprecedence over or otherwise modify the application of the technology.Also, a number of steps may be undertaken before, during, or after theabove elements are considered. Accordingly, the above description doesnot bind the scope of the claims.

As used herein and in the appended claims, the singular forms “a”, “an”,and “the” include plural references unless the context clearly dictatesotherwise. Thus, for example, reference to “a user” includes a pluralityof such users, and reference to “the processor” includes reference toone or more processors and equivalents thereof known to those skilled inthe art, and so forth.

Also, the words “comprise”, “comprising”, “contains”, “containing”,“include”, “including”, and “includes”, when used in this specificationand in the following claims, are intended to specify the presence ofstated features, integers, components, or steps, but they do notpreclude the presence or addition of one or more other features,integers, components, steps, acts, or groups.

It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this applicationand scope of the appended claims.

What is claimed is:
 1. A method of calibrating a display deviceincluding a set of primary color light sources disposed in a unit, themethod comprising: for each primary color light source of the set ofprimary color light sources, and for each target temperature of a set oftarget temperatures: setting a drive current for the primary color lightsource to a target drive current; setting drive currents fornon-characterized primary light sources of the set of primary lightsource, wherein the primary color light source is a characterized lightsource; while repeatedly measuring a temperature of the unit, modifyingthe drive currents for the non-characterized primary light sources todrive the temperature of the unit to the target temperature; andobtaining measurement points including luminance values at the targettemperature; and using the measurement points to generate a databasethat provides target drive currents for the set of primary color lightsources based on a measured temperature of the unit and a targetluminance.
 2. The method of claim 1, further comprising: storing thedatabase in a memory of the display device.
 3. The method of claim 1,wherein the database is a lookup table.
 4. The method of claim 1,wherein the set of primary color light sources are light-emitting diodes(LEDs).
 5. The method of claim 1, wherein the unit comprises a compoundparabolic concentrator (CPC).
 6. The method of claim 1, wherein themeasurement points further include chromaticity values.
 7. The method ofclaim 6, wherein obtaining the measurement points includes measuring thechromaticity values and the luminance values by receiving lighttransmitted through an eyepiece of the display device.
 8. A system forcalibrating a display device including a set of primary color lightsources disposed in a unit, the system comprising: one or moreprocessors; and a computer-readable medium comprising instructions that,when executed by the one or more processors, cause the one or moreprocessors to perform operations comprising: for each primary colorlight source of the set of primary color light sources, and for eachtarget temperature of a set of target temperatures: setting a drivecurrent for the primary color light source to a target drive current;setting drive currents for non-characterized primary light sources ofthe set of primary light source, wherein the primary color light sourceis a characterized light source; while repeatedly measuring atemperature of the unit, modifying the drive currents for thenon-characterized primary light sources to drive the temperature of theunit to the target temperature; and obtaining measurement pointsincluding luminance values at the target temperature; and using themeasurement points to generate a database that provides target drivecurrents for the set of primary color light sources based on a measuredtemperature of the unit and a target luminance.
 9. The system of claim8, wherein the operations further comprise: storing the database in amemory of the display device.
 10. The system of claim 8, wherein thedatabase is a lookup table.
 11. The system of claim 8, wherein the setof primary color light sources are light-emitting diodes (LEDs).
 12. Thesystem of claim 8, wherein the unit comprises a compound parabolicconcentrator (CPC).
 13. The system of claim 8, wherein the measurementpoints further include chromaticity values.
 14. The system of claim 13,wherein obtaining the measurement points includes measuring thechromaticity values and the luminance values by receiving lighttransmitted through an eyepiece of the display device.
 15. Anon-transitory computer-readable medium comprising instructions that,when executed by one or more processors, cause the one or moreprocessors to perform operations comprising: for each primary colorlight source of a set of primary color light sources, and for eachtarget temperature of a set of target temperatures: setting a drivecurrent for the primary color light source to a target drive current;setting drive currents for non-characterized primary light sources ofthe set of primary light source, wherein the primary color light sourceis a characterized light source; while repeatedly measuring atemperature of a unit in which the set of primary color light sourcesare disposed, modifying the drive currents for the non-characterizedprimary light sources to drive the temperature of the unit to the targettemperature; and obtaining measurement points including luminance valuesat the target temperature; and using the measurement points to generatea database that provides target drive currents for the set of primarycolor light sources based on a measured temperature of the unit and atarget luminance.
 16. The non-transitory computer-readable medium ofclaim 15, wherein the operations further comprise: storing the databasein a memory of a display device.
 17. The non-transitorycomputer-readable medium of claim 15, wherein the database is a lookuptable.
 18. The non-transitory computer-readable medium of claim 15,wherein the set of primary color light sources are light-emitting diodes(LEDs).
 19. The non-transitory computer-readable medium of claim 15,wherein the unit comprises a compound parabolic concentrator (CPC). 20.The non-transitory computer-readable medium of claim 15, wherein themeasurement points further include chromaticity values.